>
>I am a definite beginner as far as design goes. Why not tantalums?
Tantalums have very few advantages, and a lot of problems.
They like to explode when hit with too much current, and that can
easily happen with vehicle power.
>I suppose that the alternative would be ceramic?
>
>I had not checked the maximum input voltage on the 7805, Is it less
>than 13.6v? Or is there some other automotive power issue at work
>here?
400V load dump transients, inductive spikes to 30+ volts.
>Also, another sign that I know nothing, the reverse bias diode
>across the input and output of the reg. I suppose that if the input
>goes to zero while there is +5 on the output this will cause the
>damage? How big a diode do I need. A 1A rectifier rather than
>a "switching" diode, not that I know the difference except in
>current ratings and possibly switching times.... The design apps I
>have seen did not show such a diode, but it seems to make sense!
Few app notes show it these days, but it is required by almost all of
these sorts of regulators.
>As for the large electrolytics that the design may require. Would I
>need one on BOTH the input and output? I would have thought that
>one on the input would be good enough, but I can see that big
>transients may not get fed well through the reg, so a big cap on the
>output would be beneficial too....
Exactly.
I would recommend a diode in series with the input, to protect from
negative voltage transients, and also to help with your power
dissipation problem. A series fuse at 3A, and an 20V 1W zener,
reverse biased across the input, behind the fuse, should be enough to
keep your project alive. An electrolytic inside this, in front of
the regulator, will help you survive transients on the supply line.
You might consider splitting the supplies, and running your logic
from one, and the high current loads from the other.
You could then use a large enough electrolytic to power your logic
for several seconds during cranking, and let the high current devices glitch.
You'd have to deal with any consequences of that, but it would also
help spread the load off that very very warm regulator.
At 1.5A you'll need a 3A diode, and it will dissipate about 1W.
Meanwhile, your regulator will be dissipating about 14W, assuming it
is rated for 1.5A output, and it will require a large heat sink.
Worse, the ambient temperature in a car can easily pass 140F, which
makes the heatsink requirement worse. Pd = (Vin - Vout) * Iout so
(14.4 - 5) * 1.5 = 14.1W I didn't factor in the diode drop, but I
also didn't worst-case it at 15V, which pretty much balances
out. I've used 15W soldering irons, that had more heat sink area
than that regulator, if that helps put those numbers into perspective.
A switching regulator would make these heat problems pretty much disappear.
Have a look at the design guides on the national semiconductor site
for their simple switchers.
You'll be looking at a "buck" topology.